1. Field of the Invention
It can be applied more particularly to diode-pumped lasers, for example a diode-pumped laser comprising a simple coupling optical system
It can also be used as an amplifier of laser type beams.
2. Discussion of the Background
The pumping geometry of lamp-pumped lasers is almost exclusively what is called a transverse pumping geometry, i.e. the direction of propagation of the beam in the optical cavity and the greatest dimension of the lamps are parallel. The light emitted by the lamps penetrates the lasing medium through its transverse faces.
With laser diodes, whose emission is more directional than that of a lamp, it is possible to envisage another pumping geometry known as longitudinal pumping geometry. In this case, the pump beam and the laser beam get propagated in directions close to each other, and the efficiency of the laser is promoted when these two beans get superimposed with similar sections in the lasing medium. It is also possible to encourage operation in the fundamental TEM00 mode of the stable cavity when the dimension of the pumped beam is close to that of the fundamental mode.
The fundamental laser beams of stable cavities have circular sections or more or less elliptical sections that do not necessarily coincide with the shape of the emissive surfaces of the laser diodes. To obtain the superimposition of the pumped laser with the beam of the laser, there are known ways of using an optical system for the focusing or reshaping of the emitted light.
For example, the making of a longitudinally pumped laser requires the use of an optical system that focuses the light emitted by one or more high-power laser diode arrays.
FIG. 1 shows an exemplary laser diode array 1 comprising several elementary laser diodes positioned side by side. Each of the laser diodes has an emissive zone whose width lb varies from a few microns to some hundreds of microns for example, and a height hb of about one micron. The extent of the emission zone proves therefore to be highly dissymmetrical. Indeed diode arrays generally have a size of about one cm and a divergence θ// of about 10° in the plane D// parallel to the junction. In the plane D⊥perpendicular to the junction the size is in the range of one μm and the divergence θ⊥ is 50°. Thus, the extent of an array is about 2000 times greater along the plane D// than it is along the plane D⊥. The high value of the extent in the plane D// and the great dissymmetry between D// and D⊥make it difficult to design optical systems for the focusing of light emitted with the active lasing medium.
Given the present-day characteristics of high-power arrays, the light emitted may be focused on a spot with diameter of about 1 to 2 mm with efficiency values of at least 70% (the ratio between the mean power transmitted and the mean power emitted).
At output of an optical system such as this, the light is highly divergent and this is inconvenient because:                the oscillation threshold of the laser increases with the volume in which the pump energy is deposited, and,        a multimode operation may be induced if this volume is greater than the volume taken up by the fundamental mode of the cavity.        
There are known ways to reduce these phenomena by using highly doped crystals: the volume needed to absorb the pump power diminishes as the absorption coefficient increases. This approach however has certain drawbacks, for example:                it fosters the appearance of aberrations, strains and birefringence due to thermal causes: correcting these defects becomes difficult and the TEM00 mode of operation is no longer possible,        it reduces the amount of pump energy that can be deposited without reaching the damage threshold of the material, and        it limits the choice of the laser materials        
There also exist known ways in the prior art of using non-homogeneously doped lasing media.
For example, certain lasers having a transverse pumping geometry using non-uniformly doped media in order to select the desired mode.
In other applications, the geometry of the longitudinally pumped laser arrays is such that the section of the array is very small as compared with the length using the guidance of the pump light by total reflection on the periphery of the array. However, mode selection by doping is not achieved. An exemplary embodiment is given in the references E. C. Honea and al, Optics Letters, p 1203, OSA 1998 or E. C. Honea and al, Optics Letters, p 154, OSA 1999.